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<div class="refentry">
<a name="BidirectionalIterator"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Concept BidirectionalIterator</span></h2>
<p>BidirectionalIterator</p>
</div>
<div class="refsect1">
<a name="idp33343936"></a><h2>Description</h2>
<p>A bidirectional iterator is an iterator that can read through a sequence
  of values.  It can move in either direction through the sequence, and can
  be either mutable (data pointed to by it can be changed) or not mutable.</p>
<p>An iterator represents a position in a sequence.  Therefore, the
  iterator can point into the sequence (returning a value when dereferenced
  and being incrementable), or be off-the-end (and not dereferenceable or
  incrementable).</p>
</div>
<div class="refsect1">
<a name="idp33345056"></a><h2>Refinement of</h2>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p><a class="link" href="ForwardIterator.html" title="Concept ForwardIterator">ForwardIterator</a></p></li></ul></div>
</div>
<div class="refsect1">
<a name="idp33345792"></a><h2>Associated types</h2>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<p><span class="bold"><strong>value_type</strong></span></p>
<pre class="literallayout">std::iterator_traits&lt;Iter&gt;::value_type</pre>
<p>The value type of the iterator</p>
</li>
<li class="listitem">
<p><span class="bold"><strong>category</strong></span></p>
<pre class="literallayout">std::iterator_traits&lt;Iter&gt;::iterator_category</pre>
<p>The category of the iterator</p>
</li>
</ul></div>
</div>
<div class="refsect1">
<a name="idp33348192"></a><h2>Notation</h2>
<div class="variablelist"><dl>
<dt><span class="term">Iter</span></dt>
<dd>A type playing the role of iterator-type in the <a class="link" href="BidirectionalIterator.html" title="Concept BidirectionalIterator">BidirectionalIterator</a> concept.</dd>
<dt>
<span class="term"><code class="varname">i</code>, </span><span class="term"><code class="varname">j</code></span>
</dt>
<dd>Objects of type Iter</dd>
<dt><span class="term"><code class="varname">x</code></span></dt>
<dd>Object of type value_type</dd>
</dl></div>
</div>
<div class="refsect1">
<a name="idp33350496"></a><h2>Type expressions</h2>
<div class="variablelist"><dl>
<dt><span class="term">Category tag</span></dt>
<dd><p><span class="type">category</span> must be
                  derived from <span class="type">std::bidirectional_iterator_tag</span>.
                </p></dd>
</dl></div>
</div>
<div class="refsect1">
<a name="idp33351632"></a><h2>Valid expressions</h2>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>Name</th>
<th>Expression</th>
<th>Type</th>
<th>Precondition</th>
<th>Semantics</th>
<th>Postcondition</th>
</tr></thead>
<tbody>
<tr>
<td><p>Predecrement</p></td>
<td><p>--i</p></td>
<td><p><span class="type">Iter &amp;</span></p></td>
<td><p><code class="computeroutput">i</code> is incrementable (not
    off-the-end) and some dereferenceable iterator <code class="computeroutput">j</code> exists
    such that <code class="computeroutput">i == ++j</code></p></td>
<td>&#160;</td>
<td>&#160;</td>
</tr>
<tr>
<td><p>Postdecrement</p></td>
<td><p>i--</p></td>
<td><p><span class="type">Iter</span></p></td>
<td><p>Same as for predecrement</p></td>
<td><p>Equivalent to <code class="computeroutput">{Iter j = i; --i; return j;}</code></p></td>
<td><p><code class="computeroutput">i</code> is dereferenceable or
    off-the-end</p></td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect1">
<a name="idp33357184"></a><h2>Complexity</h2>
<p>
  All iterator operations must take amortized constant time.
  </p>
</div>
<div class="refsect1">
<a name="idp33357600"></a><h2>Invariants</h2>
<div class="variablelist"><dl>
<dt><span class="term">Predecrement must return object</span></dt>
<dd><p>
  <code class="computeroutput">&amp;i = &amp;(--i)</code>
  </p></dd>
<dt><span class="term">Unique path through sequence</span></dt>
<dd><p>
  <code class="computeroutput">i == j</code> implies <code class="computeroutput">--i == --j</code>
  </p></dd>
<dt><span class="term">Increment and decrement are inverses</span></dt>
<dd><p>
  <code class="computeroutput">++i; --i;</code> and <code class="computeroutput">--i; ++i;</code> must end up with the
  value of <code class="computeroutput">i</code> unmodified, if <code class="computeroutput">i</code> both of the
  operations in the pair are valid.
  </p></dd>
</dl></div>
</div>
<div class="refsect1">
<a name="idp33361632"></a><h2>Models</h2>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><span class="simplelist"><span class="type">T *</span></span></li>
<li class="listitem"><span class="simplelist"><span class="type">std::list&lt;T&gt;::iterator</span></span></li>
</ul></div>
</div>
<div class="refsect1">
<a name="idp33362912"></a><h2>See also</h2>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p><a class="link" href="RandomAccessIterator.html" title="Concept RandomAccessIterator">RandomAccessIterator</a></p></li></ul></div>
</div>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2001, 2002 Indiana University<br>Copyright &#169; 2000, 2001 University of Notre Dame du Lac<br>Copyright &#169; 2000 Jeremy Siek, Lie-Quan Lee, Andrew Lumsdaine<br>Copyright &#169; 1996-1999 Silicon Graphics Computer Systems, Inc.<br>Copyright &#169; 1994 Hewlett-Packard Company<p>Distributed under the Boost Software License, Version 1.0.
      (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at 
      <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<p>Some concepts based on versions from the MTL draft manual
      and Boost Graph and Property Map documentation, the SGI Standard
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